When a wastewater treatment facility is designed to employ batch mode influent flow treatment with a sequencing batch reactor (or SBR), the treatment plant operator realizes several advantages over the use of conventional, continuous flow treatment facilities. In an SBR system, a single vessel performs multiple functions over time, thereby placing all the necessary treatment equipment within a smaller footprint, saving the cost of purchasing and maintaining additional land. In addition, both the initial capital costs and the annual operation and maintenance cost for SBR's are ordinarily lower than continuous flow systems since there are fewer vessels to maintain and fewer pieces of equipment to install and operate.
In a basic SBR, the vessel treats a fixed volume of wastewater influent in four phases over time. The first two phases, filling and reacting, occur under dynamic flow conditions in the basin. The influent flows into the basin and is mixed with aerated activated sludge causing a reaction that captures suspended solids. These are commonly referred to as the biochemical reaction phases of the treatment process.
Following biochemical reaction, the final two phases are quiescent periods in the vessel, for clarification and sedimentation. The process of clarification and sedimentation creates an upper liquid zone and a lower solids zone in the reactor. The liquids are decanted from the top of the vessel and a portion of the solids are removed from the bottom. After separation, the liquids and solids are processed separately. In many systems, the liquids are then passed through a filtration system, such as an AquaDisk® Cloth Media Filter, to remove smaller solids that remain suspended after the prior treatment step. Solids are commonly concentrated and stored for later disposal or land application. At the conclusion of the fourth phase the vessel is again ready to treat another volume of the influent.
In a typical SBR, since the reaction and clarification phases occur in the same vessel, the size of the vessel will be selected based on the phase or process step that requires the largest volume, making the basin oversized for the remaining phases. Since aeration is intermittent, the aeration equipment (blowers and diffusers) is commonly over designed to deliver the needed oxygen to the activated sludge within the time allotted for biochemical reaction. The inability to aerate a vessel continuously means that, maximum oxygen utilization rates cannot be accommodated in small reaction tanks.
Each SBR vessel can be “turned down” to some extent so as to be operated at less than its full influent volume capacity. However, the turn down ratio is limited to a fixed fraction of the full volume. Therefore, the larger the vessel size, the greater the initial flow requirement will be for operation in a turned down condition. Turned down operations are particularly important during the initial operation of an SBR. Vessel sizes are normally determined based on projected influent volumes that are expected ten to twenty years into the future. When these SBR vessels are first operated, incoming flows may be lower than the full turned down capacity of the system requiring storage of the influent until the minimum volume is available. By reducing the size of each SBR vessel, smaller incoming flows can be treated.
In normal operation, the water level in an SBR vessel rises and falls with each batch cycle. The rising and falling water line makes it more difficult to remove floatable pollutants like oils and greases (generically referred to as “scum”) in comparison to a vessel that operates at a fixed water level. The present inventions overcome these and other limitations of current SBR treatment processes.
It is a feature and an advantage of the present inventions that the reaction phases are separated from the clarification phases by providing separate vessels that operate independently of each other. Such separation—while still operating each vessel in a batch mode—optimizes the vessel volumes for each treatment phase. Optimized volumes result in smaller land footprints, lower construction costs and higher cost effectiveness for SBR systems.
With a separate biochemical reaction vessel, reaction time with the present invention may increase to 24 hours per day. It is a feature and an advantage of the present inventions that this improvement allows for aeration equipment to be reduced in size, thereby reducing equipment and capital cost for SBR systems. It is a further feature and advantage of the present inventions that continuous aeration accommodates maximum oxygen utilization rates (OURs) in smaller tanks than conventional SBR systems.
It is also a feature and advantage of the present inventions that the smaller operating vessels resulting from the implementation of the inventions allow for a greater range of turn down capabilities. Increased turn down ranges allow treatment plant operators to handle initial flows to a new SBR that would be too low to properly treat with a conventional SBR system.
It is also a feature and an advantage of the inventions that, unlike conventional SBR systems, the clarification component of the inventions can operate at a fixed water level to better facilitate scum removal.